Replies

1. Expert Member
   Michael Maines | Feb 15, 2020 01:48pm | #1

   Andy, are you planning on doing a floating subfloor over the EPS, or placing EPS between sleepers fastened to the slab?

   If it's a sleeper system, 10 psi EPS will be fine.

   In a floating system, 10 psi EPS might be cutting it close. It is rated for the maximum load that results in 10% deflection, and good practice is to plan for 1/3 load that for long-term creep under sustained loads. You may not expect to have long-term loads, but furniture, bookshelves etc. are included in that category.

   In everyday use, the heaviest live loads would be a person walking--say 250 lbs over 10 square inches. The load spreads out as it transfers through the subfloor, to about 20 square inches. 250 lbs on 20 sq. in. is 12.5 psi, or about 25% more than the rated load. Deflection in foam is fairly linear (except at initial loading and near failure) so that just means 25% more deflection. If you are using 2" foam, 12.5% deflection is 1/4".

   Under more typical design loads of 45 per square foot (40 psf plus 5 psf dead load), or more like 200 square inches once the load transfers through the subfloor, you'll have a load of about 0.23 psi. (45 lbs / 200 in²). Increase that by 3X to account for long-term creep, and the total is still below 1 psi.

   What this math shows is that when the subfloor adequately spreads out any point loads, 10 psi foam should work, with a factor of safety of more than 10. If the subfloor does not do a good job of spreading out the load, then 10 psi could result in excessive deflection. I play it safe and recommend 15 psi EPS under floating subfloors. Others play it even safer, with 25 psi foam, but I think that's overkill except when under structural loads.

   15 psi EPS usually has to be special ordered. I get it from a company a few states away, on this list: foam-control.com/contact-us-foam-control/. They deliver it to the job site, usually within a week or two at most, for a total cost comparable to buying climate-damaging XPS locally.

   Even better than EPS would be to find a local source of recycled foam. XPS is almost always 15 psi or higher. Just don't use polyiso in that location, due to risk of water absorption.

1. 
   AndyNelson | Feb 15, 2020 02:58pm | #2

   Hi Michael-

   Thanks for the reply! I'm planning to glue the EPS to the concrete floor and then fasten the plywood to the foam+concreate using tapcon screws. I will likely do vinyl plank flooring over the plywood.

   Andy

1. 
   AndyNelson | Feb 21, 2020 12:03pm | #3

   Quick bump on this - I assume my assembly mentioned above would function the same as the 'sleepers fastened to slab' example that Michael gave? In my area, the local insulation supply houses largely carry 25 psi EPS and their pricing is not close to the big box store 10 psi EPS - so, if I can, I would like to use the 10 psi EPS on the floors and save some money. Thanks all!!

2. Expert Member
   Dana Dorsett | Feb 21, 2020 12:51pm | #4

   >"Most information I see on this site suggests at least 15 PSI. Is 10 PSI sufficient for my application?"

   Where do you find that suggestion? What rationales or explanations are offered to support that?

   With a double layer of half-inch (seams staggered ) or a single layer of 3/4" there aren't any issues with 10 psi (@ 10% deformation) rated foam, as long as the seams of single-layered plywood don't coincide with the seams of the foam, with a foot or so of separation.

   Think about it- 10 psi is 1440 lb per square foot. With at least 3/4" of plywood to distribute the load there are no residential applications (except under structural walls) that even come close to those numbers unless you're regularly hammering hard on the plywood with a sledge.

   https://www.epsindustry.org/building-construction/compressive-strength

1. 
   AndyNelson | Feb 21, 2020 01:16pm | #5

   Thanks Dana. My comment was more anecdotal via some of the other posts I was reading. It felt like most of the comments were recommending 15 psi.

   Due to height limitations, I was planning to go with 3/4" EPS and a 1/2' plywood (or maybe OSB, haven't yet) but I get the sense that I should be fine. You're explanation of 1440 lb per square foot helped make it more clear and obvious.

   Thanks!
   Andy

1. 
   nbatalla | May 29, 2024 04:31pm | #6

   Andy,

   I have been pouring over as much content as I can stomach, and this is the only content on the site that I've been able to find answering if 10 psi eps foam is strong enough. Did you go ahead and use 10 psi eps foam for your basement floor assembly?

   I value Dana's answer and perspective on this, unfortunately I can't seem to find any other content going down this route.